Rapid production apparatus with production orientation determination

ABSTRACT

A method of producing an object by sequentially printing layers of construction material one on top of the other, the method comprising: providing the construction material at a first lower temperature; flowing the construction material through a heated flow path in a flow structure to heat the construction material and delivering the heated construction material to a heated reservoir in a printing head; and dispensing the heated construction material from the reservoir to build the object layer by layer.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/330,073 filed on Jul. 14, 2014, which is a continuation of U.S.patent application Ser. No. 13/541,811 filed on Jul. 5, 2012, now U.S.Pat. No. 8,781,615, which is a division of U.S. patent application Ser.No. 12/529,377 filed on Apr. 11, 2010, now U.S. Pat. No. 8,219,234,which is a National Phase of PCT Patent Application No.PCT/IL2007/000286 having International Filing Date of Mar. 7, 2007. Thecontents of the above applications are all incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to apparatus, hereinafter “rapidproduction apparatus”, (RPA) for producing a 3-dimensional object bysequentially forming thin layers of material, one on top of the other,responsive to data defining the object.

BACKGROUND OF THE INVENTION

Rapid production apparatus (RPAs) form objects by sequentially formingthin layers, hereinafter “construction layers”, of construction materialone on top of the other responsive to data, hereinafter “constructiondata”, defining the objects. There are numerous and varied types of RPAsand different methods by which they form the thin construction layersfrom which they build an object.

One type of RPA, conventionally referred to as an “ink-jet RPA”,“prints” each layer of an object it builds. To form a given layer, theink-jet RPA controls at least one dispenser, referred to as a “printinghead”, to dispense at least one construction material (CM), hereinaftera “building material” (BM), in liquid form in a pattern responsive toconstruction data for the object and then solidifies the dispensedmaterial. Generally, the layer is printed in the shape of a crosssection of the object. Building material in adjacent, contiguousconstruction layers is printed in the shape of thin cross sections ofthe object that are displaced relative to each other by a smallincremental distance along a same direction, hereinafter referred to asa “stacking direction”, relative to the object. The incremental distanceis generally equal to the thickness of the layers.

For convenience of exposition, the cross sections of the object in whoseshapes the construction layers are formed are assumed to be parallel tothe xy-plane of a suitable coordinate system and the stacking directionis in the z-direction of the coordinate system. Optionally, the buildingmaterial is a photopolymer, which is hardened after dispensing byexposure to suitable electromagnetic radiation, typically UV radiation.

For many construction objects, because of the complexity and/or shape ofthe objects, construction layers comprising only BM printed in the shapeof cross sections of the construction objects are not completelyself-supporting and require support during construction of the object.For such cases, at least one additional construction material CM,hereinafter referred to as “support material” (SM) is printed asrequired in suitable regions of each layer, to provide support for thebuilding material in following layers. The support material and/or ashape in which it is formed, is such that upon completion of the objectit can be removed from the object without substantially damaging thebuilding material. In some embodiments, the support material, like thebuilding material, is also a photopolymer.

An ink-jet type of RPA typically comprises at least one ink-jet printinghead comprised in a “printing head block” that is mounted to a“shuttle”. Each printing head has an array of one or more outputorifices and is controllable to dispense construction material, BMand/or SM, from each orifice independently of dispensing constructionmaterial from the other orifices. The construction material generallycomprises one or more types of photopolymer materials typically storedin at least one supply cartridge. A suitable configuration of pipes andpumps transports the material or materials from the at least one supplycartridge to one or more reservoirs in the printing head block fromwhich the at least one ink-jet printing head receives the constructionmaterial. Optionally, to maintain appropriate viscosity of the at leastone photopolymer, a controller controls at least one heater, optionallymounted to the printing block, printing head and/or reservoir, to heatthe photopolymer to a suitable operating temperature. The one or moretypes of photopolymers may, generally, be dispensed in any combination,separately or together, simultaneously or consecutively.

During construction of an object, a controller controls the shuttle torepeatedly move over a support surface, hereinafter a “constructionplatform”, parallel to the x-y plane on which the object is formed bythe RPA. The support surface and shuttle are generally housed in an atleast partially enclosed “construction hangar” that protects the objectwhile it is being built. As the shuttle moves, the controller controlseach printing head to dispense construction material selectively throughits orifices responsive to construction data defining the object, toprint one construction layer of the object on top of the other on theconstruction platform.

Optionally, in moving the shuttle over the support surface duringproduction of a construction layer, the controller controls the shuttleto move back and forth along a “scanning direction” conventionallydefined as the x-direction. Optionally, at any one or more reversals ofthe shuttle along the x-direction, the controller incrementsdisplacement of the shuttle in a direction perpendicular to the scanningdirection, in a direction referred to as “displacement” direction,conventionally the y-direction. Following production of a givenconstruction layer, either the construction platform is lowered or theshuttle raised, along the stacking direction by a distance equal to athickness of a next construction layer to be produced over the justformed given layer.

Mounted to the shuttle, adjacent to the printing head block are one ormore sources of electromagnetic radiation, optionally UV radiation, forcuring the photopolymer construction material printed in eachconstruction layer. Also, optionally, mounted to the shuttle adjacent tothe at least one printing head block is a “leveling roller” which levelsnewly printed layers of construction material to a predetermined layerheight by removing surplus material from the layer. The surplus materialremoved from the layer is wiped off the roller by a “cleaning wiper” andgathered in a waste container comprised in the shuttle. Alternatively,the support surface moves.

Configurations of ink-jet type RPAs are described in U.S. Pat. No.6,193,923 (Leyden), U.S. Pat. No. 6,259,962, U.S. Pat. No. 6,658,314,U.S. Pat. No. 6,569,373, U.S. Pat. No. 6,850,334 and U.S. applicationSer. Nos. 10/716,426, 10/336,032 and PCT Publication WO 2004/096527, thedisclosures of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention, relates to providingimproved control of dispensing BM and/or SM material by a printing headin the RPA by providing improved control of the temperature of dispensedmaterial.

Temperature of a BM or SM material dispensed by a printing head tends todecrease as a rate at which the printing head dispenses the materialincreases, because a time period available to a conventional RPA heaterto bring the material to a desired temperature decreases as dispensingrate increases. The temperature decrease in general adversely affectsviscosity of the material and thereby its ability to flow and form ahomogenous, quality construction layer that bonds properly to adjacentconstruction layers. If dispensing rates vary during production of anobject or objects, it is generally difficult to compensate for reducedheating time by controlling temperature of the heaters.

In accordance with an embodiment of the invention, an RPA is providedwith a heating unit hereinafter referred to as a “flow-heater” or “preheater” that substantially reduces dependence of temperature at which BMor SM is dispensed by its printing head on a rate at which the materialis dispensed. In accordance with an embodiment of the invention, theflow-heater comprises a heat sink having a relatively large thermalcapacity that is formed with an internal flow channel through whichmaterial dispensed by the printing head flows to reach the reservoir inthe printing head. A heating element provides heat to the heat sink tomaintain the temperature of the heart sink at a temperature at which itis desired that the BM or SM should be dispensed. The flow channel ismade sufficiently long so that for a range of material dispensing ratesat which the RPA operates, during passage of the material through thechannel, temperature of the material equalizes to that of the heat sink.In some embodiments of the invention a pre heater is supplemented orreplaced by a flow-heater that heats the BM or SM material after itleaves the reservoir.

An aspect of some embodiments of the invention relates to a method andapparatus for removing finished objects from an RPA constructionplatform.

In general, before beginning construction of an object (or objects), anRPA lays down a “foundation” on the RPA construction platform. Theobject is then built by depositing layers of BM or SM on the foundationand planning off the deposited layers with an RPA leveling roller. Thefoundation provides a relatively accurately planar surface parallel tothe x-y motion of the RPA printing head block on which to build theobject and provides material between the object and the surface of theconstruction platform that enables the object to be scraped off theconstruction platform without damaging the object. Conventionally, eachfinished object is removed by scraping and/or peeling the foundation offthe construction platform using a conventional scraping tool such as apaint scraper. Whereas to save construction costs and time it isadvantageous for the foundation to be as thin as possible, thefoundation is formed sufficiently thick so that it and the object may bescraped or lifted off the construction platform without damage to theobject.

In accordance with an embodiment of the invention, a “spatula shovel” isprovided for removing finished objects from the construction platform.The spatula shovel comprises a relatively thin plate formed with astraight, relatively sharp leading edge and optionally has a length andwidth substantially equal respectively to the length and width of theconstruction platform. Optionally, the structural shape of the spatulais reinforced by upturned edges. The sharp front edge of the spatulashovel is forced between the foundation and the construction platform topeel and/or scrape and lift off the foundation and finished objects fromthe construction platform and functions as a support and carry tray forthe objects once they are lifted off the construction platform. Theleading edge is sufficiently sharp and the plate sufficiently thin yetrigid so that the objects are relatively easily removed without damage.

The inventors have found that using a spatula shovel in accordance withan embodiment of the invention to remove finished objects from aconstruction platform generally enables the objects to be removed moreeasily and with less chance of damage than by using a conventionalscraping tool. As a result, a foundation on which objects are formed maybe made substantially thinner than conventional foundations. The thinnerfoundation provides savings in production materials and objectproduction time.

An aspect of some embodiments of the invention relates to providingdifferent production modes for an object printed by the RPA so that auser of the RPA can tailor production time and quality of an object tothe user's needs.

In accordance with an embodiment of the invention, an RPA is configuredto operate selectively in different production modes chosen from aplurality of production modes. Optionally, the plurality of productionmodes comprises three production modes: a draft mode, a standard modeand a high quality mode. For each of the different production modes,production parameters that determine RPA operation and affect qualityand rate of production of an object are adjusted as required to assumevalues that correspond to the production mode and are compatible witheach other.

Cost of an object produced by an RPA depends upon amounts of materialused in producing the object and its production time. Whereas, ingeneral there is little control over an amount of BM required to producean object, because the amount is determined by the object's volume, anamount of SM required to produce the object depends, in general, uponthe object's orientation on the RPAs production platform.

An aspect of some embodiments of the invention relates to providing analgorithm for determining orientation of an object to be built on an RPAconstruction platform in order to reduce an amount of SM used in itsproduction.

Production time of an object or objects is a function, inter alia, of anumber of scan passes, i.e. passes along, optionally, the scanning,x-direction, that the shuttle makes in order to produce the object orobjects, and the lengths of the scan passes. The number and lengths ofthe scan passes are functions of orientation and relative position ofthe objects on the construction platform.

An aspect of some embodiments of the invention relates to a method fordetermining relative orientation of objects to be built by an RPA, andrelative position of the objects on the RPAs construction platform inorder to reduce the number and lengths of scan passes used to producethe objects.

Hereinafter, an orientation on an RPA construction platform of a singleobject to be produced by the RPA and a configuration of orientations andrelative positions of a plurality of objects to be produced on the RPAconstruction platform are referred to as “build configurations”.

An aspect of some embodiments of the invention relates to providingmethods of calibrating a printing head block so that CM material isaccurately deposited at desired locations during production of anobject.

To control CM deposition so that the CM is deposited accurately atdesired locations during production of an object, position of an RPAshuttle during its motion along scanning and displacement directionsshould accurately be known and timing of activation of printing headsmounted in the shuttle to dispense CM material should accurately becoordinated with the shuttle motion and position. In accordance with anembodiment of the invention, calibration is performed to determine ifprinting heads are misaligned relative to each other and/or to scanningand displacement directions and to correct and/or compensate formisalignment. In an embodiment of the invention, calibration isperformed responsive to at least one test pattern printed by the RPA.Optionally, the at least one test pattern comprises a plurality ofstraight lines.

Optionally a determined misalignment is corrected, at least in part, bymechanically realigning the printing head and/or the printing head blockin which it is mounted and/or the shuttle that comprises the printinghead block. Optionally, misalignment is compensated for, at least inpart, by adjusting relative timing of command signals transmitted to atleast one printing head to control dispensing material from an orificeor orifices of the at least one printing head.

There is thus provided, in accordance with an embodiment of theinvention, a method of producing an object by sequentially printinglayers of construction material one on top of the other, the methodcomprising:

providing the construction material at a first lower temperature;

flowing the construction material through a heated flow path in a flowstructure to heat the construction material and delivering the heatedconstruction material to a heated reservoir in a printing head; and

dispensing the heated construction material from the reservoir to buildthe object layer by layer.

Optionally, the method includes controlling one or more of the flow rateand the temperature of the flow structure to provide a desiredtemperature of construction material to the reservoir.

In an embodiment of the invention, the temperature and length of theflow structure is high enough so that the temperature of theconstruction material leaving the reservoir remains substantiallyconstant up to a flow rate of at least 5 gm/sec. Optionally, thetemperature of the construction material leaving the reservoir remainssubstantially constant up to a flow rate of at least 5 gm/sec.

Optionally, the method providing a controller that controls thetemperature of the flow structure such that the temperature of the flowstructure changes with flow rate to assure the substantial constancy ofthe temperature of construction material leaving the reservoir.

Optionally, the method forming the flow path as a channel in a heat sinkheated by a heater. Optionally, the at least one channel is spiralshaped.

In an embodiment of the invention, the flow path is provided as ameandering path at least a portion of which is heated.

In an embodiment of the invention, the flow path is located inside thereservoir. Optionally, the heating unit of the flow path is mounted tothe reservoir. Optionally the heating units of the flow path and of thereservoir are the same unit.

There is further provided, in accordance with an embodiment of theinvention, apparatus for producing an object by sequentially printingthin layers of a construction material one on top of the other in astacking direction, the apparatus comprising:

at least one printing head arranged for relative motion with respect toa surface on which the object is to be constructed and having orificesfrom which the construction material is dispensed to print the layers;

a controller having a plurality of selectable operation modes;

an interface configured to receive input indicating a selected operationmode and transmit data indicating the selection to the controller;wherein

the controller controls the relative motion and/or the operation of theat least one printing head and/or the layer thickness responsive to theselected operation mode.

Optionally, the production modes comprise at least two of draft,standard and high quality production modes.

Optionally the controller sets at least one production parameter thataffects production to satisfy a specification of the selected productionmode. Optionally, the at least one production parameter comprises atleast one of: a rate of depositing construction material; scanningvelocity; UV power for curing construction material; flow rate of airfor cooling deposited construction material; construction layerthickness.

There is further provided, in accordance with an embodiment of theinvention, a method for producing an object by sequentially printingthin layers of construction material the layers being printed one on topof the other in a stacking direction on a surface of a constructionplatform, the method comprising:

adding support construction for supporting down facing facets of theobject;

orienting the object in respect to the construction platform in a waythat the volume of supporting structure is minimum, and

printing the object on the platform so that it is oriented in thedetermined build configuration.

In an embodiment of the invention, the orientation of minimum support isapproximated by the orientation of lowest center of mass of the objectin respect to the construction platform

Optionally, orientation of minimum support is approximated by:

orienting the object on the platform at a first orientation a;

rotating the object about an X axis by + and −90° and denoting thoseorientations as orientations b and c respectively;

rotating the object from orientation a about a Y axis by + and −90° anddenoting those orientations as orientations d and e respectively;

rotating the object from orientation a about Y axis +180° and denotingthat orientation as orientation f;

computing the volume of supporting structure for each of orientations ato f;

determining the orientation for placement of the object on the platformas one of orientations a through f requiring minimum support.

There is further provided, in accordance with an embodiment of theinvention, a method for producing an object by sequentially printingthin layers of build material, the layers being printed one on top ofthe other in a stacking direction on a surface of a constructionplatform, the method comprising:

determining a plurality of surface vectors each of which isperpendicular to a surface region of the object;

defining groups of surface vectors for which an angle between any twovectors in the group is less than a predetermined maximum angle;

defining for each group of surface vectors a group vector having amagnitude equal to a sum of the absolute values of the surface vectorsin the group and a direction substantially parallel to at least one ofthe surface vectors;

determining a production orientation of the object responsive to thegroup vectors; and

printing the object on the platform so that it is oriented in thedetermined production orientation.

Optionally, the predetermined maximum angle is less than or equal toabout 1° or about 5°.

In an embodiment of the invention, printing comprises depositingconstruction material in a first pass parallel to a scanning directionand, as required, in an additional pass parallel to the first pass anddisplaced therefrom along a displacement direction.

In an embodiment of the invention, determining the productionorientation comprises orienting the object so that a maximum groupvector is parallel to one of the three directions: scanning (X),displacement (Y), and stacking (Z). Optionally, the method includesorienting the object so that a next largest vector is parallel to one ofthe rest of X, Y, Z directions.

There is further provided, in accordance with an embodiment of theinvention a method for producing an object by sequentially printing thinlayers of build material, the layers being printed one on top of theother in a stacking direction on a surface of a construction platform,the method comprising determining a production orientation for theobject for which a rectangular parallelepiped having a volume that justcontains the object and for which each face of the parallelepiped isperpendicular to one of the stacking, scanning and displacementdirections, has a minimum volume.

In an embodiment of the invention, the method includes:

determining a coordinate system fixed to the object and having axesparallel to the stacking, scanning and displacement directions; and

orienting the object in a plurality of different orientations for whicheach axis is parallel to one of the stacking, scanning and displacementdirections.

Optionally, building the objects includes adding supporting structure,and determining for each of the plurality of orientations an amount ofsupport material required to produce the object in the orientation.Optionally, the method includes determining the production orientationto be that orientation of the plurality of orientations, for whichsupport material is a minimum.

There is further provided, in accordance with an embodiment of theinvention, a method of simultaneously producing a plurality of objectsby sequentially printing thin layers of construction material one on topof the other in a stacking direction Z on a surface of a constructionplatform, the method comprising:

determining a build configuration for producing the objects whichdefines a position and orientation of each of the objects on theplatform; and

producing the objects by printing layers of construction material, eachlayer being printed in a series of scans along a scanning direction andbeing mutually displaced in a direction Y;

wherein at least two said scan series have different positions of thescans in the Y direction responsive to the layer Y geometry.

Optionally, at least three layers are produced using scan series havingdifferent positions of scans in the Y direction.

Optionally, the method includes determining a position for an objectsuch that the object's projection on the Y-Z plane is included in thesum of the projections of previously placed objects on the Y-Z plane.Optionally, the method includes determining a position for an objectcomprising:

determining the projection on the Y-Z plane of the object;

ordering the objects responsive to the dimensions of their Y-Zprojections.

Optionally, the method includes determining a position for an object,such that the object's projection on the Y-Z plane includes the sum ofthe projections of previously placed objects on the Y-Z plane.

There is further provided, in accordance with an embodiment of theinvention, a method of calibrating a rapid production apparatuscomprising a printing head block mounted with a plurality of printingheads, each having an array of orifices equally spaced along arespective printing head axis, from which orifices construction materialis deposited and wherein the arrays are parallel to each other andconstruction material is printed during motion of the block along ascanning x-direction, the method comprising:

first printing at least one dot of material from at least one orifice ofone of the printing heads;

then printing a second at least one dot of material from an orifice of asame or different printing head;

determining a distance between the two at least one dots;

adjusting or calibrating the apparatus responsive to the distance.

Optionally, the distance is less than 5 mm.

Optionally, the dots are printed on a transparent substrate.

Optionally, the two heads are different from each other, the distance ismeasured along X direction, and the distance is used to calibrate thejetting X distance between said heads.

Optionally, the distance is measured along the X direction, and thedistance is used to aligning the head's block so that the heads aresubstantially parallel to Y axis.

In an embodiment of the invention the two heads are the same head, wherethe first at least one dot is jetted by one of the lower nozzles of thehead and the second at least one dot is jetted by one of the uppernozzles of the head, where between the first and second jetting of thetwo at least one dots the block is moved along the indexing Y direction.Optionally, the distance is measured along Y direction, and the distanceis used to calibrate Y movement.

In an embodiment of the invention, the first at least one dot is jettedby one head and the second at least one dot is jetted by a second head,where between the first and second jetting of the two at least one dotsthe block is moved along the scanning X direction, where said distanceis measured along Y direction, and the distance is used to aligning thehead's block so that the heads are substantially perpendicular to Xaxis.

There is further provided, in accordance with an embodiment of theinvention, a method of removing an object produced by the layer wiselaying down of construction material on a construction platform, theconstruction material being laid down on a thin base of material, themethod comprising:

providing a shovel having a flat lifting surface at least as wide as anextent of the object along the base;

sliding the flat surface under the object, while keeping the flatsurface parallel to the construction platform; and

lifting the object from the construction platform.

Optionally, at least a portion of the base is lifted together with theobject.

Optionally, the flat surface is less than 1 mm thick.

Optionally, the shovel is formed with stiffening edges.

Optionally, the flat surface is formed with a feathered leading edge.Optionally, the feathered leading edge is composed of one single slopefrom the upper surface of said flat surface to the lower surface of theflat surface.

There is further provided, in accordance with an embodiment of theinvention, a method of simultaneously producing a plurality of objectsby sequentially printing thin layers of construction material one on topof the other in a stacking direction Z on a surface of a constructionplatform, the method comprising:

determining a build configuration for producing the objects whichdefines a position and orientation of each of the objects on theplatform; and

producing the objects by printing the construction material in a firstpass parallel to a scanning direction X and, as required, in anadditional pass parallel to the first pass displaced therefrom along adisplacement direction Y so that each object is in the position andorientation of the build configuration;

wherein the Y coordinate of at least one pass on one layer is differentfrom that of any of the passes on a lower layer.

Optionally, objects having a requirement for printing upper layers areso aligned that a minimum number of passes is required to print theupper layers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto, which arelisted following this paragraph. In the figures, identical structures,elements or parts that appear in more than one figure are generallylabeled with a same symbol in all the figures in which they appear.Dimensions of components and features shown in the figures are chosenfor convenience and clarity of presentation and are not necessarilyshown to scale.

FIG. 1A schematically shows a rapid production apparatus (RPA) inaccordance with an embodiment of the present invention;

FIG. 1B schematically shows a flow-heater which can be incorporated inthe RPA shown in FIG. 1A, in accordance with an embodiment of thepresent invention;

FIG. 1C schematically shows a CM reservoir including a flow-heatercomprised in a printing head block, in accordance with an embodiment ofthe invention;

FIG. 1D schematically shows another CM reservoir in accordance withembodiments of the invention;

FIG. 2 schematically shows a spatula shovel being used to remove objectsfrom a construction platform, in accordance with an embodiment of theinvention;

FIG. 3 schematically shows a bottom perspective view of a shuttle, whichmay be comprised in the RPA shown in FIG. 1A, in accordance with anembodiment of the present invention;

FIGS. 4A-4C schematically show test patterns used to calibrate an RPA,such as the RPA shown in FIG. 1A, in accordance with an embodiment ofthe invention;

FIGS. 5A and 5B show a flow diagram of a method for determining a buildconfiguration for producing an object that requires a relatively smallamount of SM, in accordance with an embodiment of the invention;

FIGS. 6A-6M schematically illustrate orientations of an object thatcorrespond to and illustrate acts in the method shown in FIGS. 5A and5B;

FIGS. 7A and 7B schematically show features of an object that are usedin determining a build configuration for a plurality of objectscomprising the object, in accordance with an embodiment of theinvention;

FIG. 7C demonstrates the advantage of increased similarity brought aboutby “preferred orientation” in reducing the average Y span of two parts,in accordance with an embodiment of the invention; and

FIGS. 8, 9A and 9B illustrate placement of various height objects inaccordance with an embodiment of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

FIG. 1A schematically shows an ink-jet RPA 20 producing one or moreobjects 22, on a construction platform 24, in accordance with anembodiment of the invention. By way of example, objects 22 comprisethree objects: a hollow box, a vase and a cup, which are shown partiallyconstructed in FIG. 1A.

RPA 20 comprises a controller 26 and a shuttle 28 comprising a printinghead block 50, a leveling roller 27 and, optionally, two sources 120 ofradiation suitable for polymerizing photopolymer CM used by the RPA toconstruct objects. Printing head block 50 comprises at least one ink-jetprinting head (not shown in FIG. 1A), optionally having a plurality ofoutput orifices configured in a linear array. The at least one printinghead is controllable to dispense, i.e. print, construction material CM,e.g. build material BM and/or support material SM, from each of itsorifices independently of dispensing construction material from theother orifices. Optionally, photopolymers used by RPA 20 are UV curableand radiation sources 120 are UV lamps. Optionally, only the buildmaterial is curable and the support material is a hot melt material.Optionally, construction platform 24 is mounted to a support structureor table and is controllable to be lowered and raised with respect tothe support structure or table and thereby with respect to shuttle 28using any of various methods known in the art. RPA 20 is shown veryschematically and only features and components of the RPA germane to thediscussion are shown in FIG. 1A. A coordinate system 21 is used toreference locations and positions of features and components of RPA 20.

To produce objects 22, controller 26 first controls shuttle 28 to moveover construction platform 24 parallel to the x-y plane. While theshuttle is in motion, the controller controls the printing heads inprinting head block 50 to dispense and lay down, i.e. print, on platform24 a plurality of optionally SM construction layers to form a foundation35 on platform 24 that is accurately planar and parallel to the x-yplane on which to build each object 22. Optionally a single largefoundation is built on which to construct the objects. Optionally, aplurality of foundations, which may comprise a different singlefoundation for each object, is built. By way of example and forconvenience, FIG. 1 shows a single large foundation 35 on platform 24 onwhich objects 22 are built.

Optionally, in printing construction layers, controller 26 controlsshuttle 28 to move back and forth parallel to a scanning direction,optionally parallel to the x-axis, in directions indicated by adouble-headed block arrow 31. Following one or more reversals ofdirection along the x-axis, the controller, as needed, advances shuttle28 along a displacement direction indicated by a block arrow 32,optionally, parallel to the y-axis. Optionally, printing head block 50is stationary and platform 24 moves in the x and y directions to scanthe objects 22 with respect to head 50.

After construction material is freshly dispensed to form a region of agiven construction layer (or the entire layer) in foundation 35 levelingroller 27 (or other leveling device as known in the art) contacts theregion, and in accordance with any of various methods known in the artflattens and levels it to a desired thickness by shaving off orotherwise removing an upper portion of the printed material. Thematerial in the freshly dispensed and leveled layer is then hardened bycuring with UV light from UV lamps 120. Optionally, the constructionmaterial is partially hardened before leveling. After foundation 35 isformed, the controller controls shuttle 28 to move over constructionplatform 24 to dispense, level and cure construction material, BM,and/or support material, SM, as required and form construction layers 34that are used to produce objects 22 on the foundation.

Construction layers in foundation 35 and construction layers 34 arestacked in a direction, i.e. a stacking direction perpendicular toconstruction platform 24 and parallel to the z-axis. Following formationof a given construction layer 34, optionally, construction platform 24is lowered by a distance substantially equal to a thickness of a nextconstruction layer to be formed on the given construction layer. Forconvenience of presentation, thickness of construction layers 34 isgreatly exaggerated in FIG. 1A. Typically, construction layers 34 arebetween 8 and 60 micrometers thick.

Construction layers 34 are produced responsive to digital constructiondata configured in any of various formats, for example three-dimensionalSLA, known in the art. By way of example, the vase being constructed byRPA 20 is a copy of a vase 36 shown in an inset 38, which is defined bydigital construction data. Inset 38 schematically shows vase 36 formedfrom “data cross-section” layers 40 that are defined by the vase'sconstruction data. RPA 20 forms construction layers 34 in the vase beingproduced on platform 24 responsive to data cross-section layers 40. Thedata cross-section layers are optionally defined by controller 26 fromthe digital construction data that defines vase 36 or are alreadydefined by the construction data. A block arrow 42 schematicallyindicates that the construction data is input to controller 26 andsuitably processed by the controller to control production ofconstruction layers 34.

In order for CM material printed by printing heads in printing headblock 50 to flow properly and form a homogeneous layer of material thatbonds properly to a layer on which it is deposited, the CM must ingeneral be maintained at an appropriate dispensing temperature.Dispensing temperatures for common construction materials CM varybetween about 60° C. and 80° C. Conventionally, a suitable supply systemprovides relatively cold, generally room temperature (or somewhat heatedto facilitate flow) CM to shuttle 28 and its printing head block 50, andheaters mounted to the printing head block, in the printing heads and/orin a reservoir in the printing head block, are used to control CMdispensing temperature. However, during production of an object orobjects such as objects 22, under conditions for which a CM dispensingrate may undergo substantial changes, it can be difficult to maintain adesired dispensing temperature using a conventional configuration ofheaters. A rate at which RPA 20 dispenses a CM may vary from arelatively low rate close to zero to a relatively high rate of about 0.4or about 0.5 gr/s.

In general there is a relatively large mass of CM in the reservoir. Whenthe flow rate is relatively low, the heater of the reservoir issufficient to keep the temperature within an optimal range. However,when the flow rate from (and to) the reservoir is increased thetemperature of the CM in the reservoir goes down and can fall below theoptimal range. Increasing the power input into the reservoir is not apractical solution to this problem since the thermal coupling of theheater and the bulk of the fluid in the reservoir is not sufficient.

In accordance with an embodiment of the invention, to provide desiredcontrol of the dispensing temperature of construction material CM duringproduction, RPA 20 is equipped with at least one heating unit, i.e. aflow-heater 100, optionally mounted to shuttle 28. Flow-heater 100operates to control temperate of CM dispensed by RPA 20 at a desiredtemperature for substantially all rates at which printing heads inshuttle 28 dispense CM. Details of suitable, non-limiting examples ofsuitable flow heaters, in accordance with embodiments of the invention,are shown in FIGS. 1B-1D. It should be understood that flow heatersaccording to some embodiments of the invention differ from the heatingof reservoirs according to the prior art, in that the flow heaters ofthe present embodiments provide a more intimate transfer of heat to theflowing material, which enables better control of the heating of theflow material.

The position of a suitable flow-heater 100, in accordance with anembodiment of the invention, is shown in FIG. 1A. The internal structureis shown schematically shown in FIG. 1B.

Flow-heater 100 comprises a heat sink 102 having relatively largethermal capacity and thermal conductivity made from a suitable materialsuch as aluminum and at least one heating element 104 controllable toprovide heat that maintains the temperature of the heat sink at adesired temperature. By way of example, heat sink 102 is shown as arectangular block of material and at least one heating element 104 isshown as two heating elements, each covering a face of the heat sink.Heat sink 102 is formed with at least one relatively long and optionallynarrow internal flow channel 106 shown in dashed lines. Optionally, inorder to provide at least one flow channel 106 with a desired length, toenable efficient heat transfer to the flowing material, the flow channelis spiral shaped. By way of example, heat sink 102 is formed with twoflow channels 106.

Controller 26 (FIG. 1A) controls a suitable supply system to providerelatively cold, generally room temperature CM to each flow channel 106optionally via an input pipe 108 so that the CM passes through the flowchannel to enter shuttle 28, optionally via an exit pipe 110. Optionallya different CM flows through each channel 106 (e.g. BM may flow throughone channel and SM through the other). Dimensions of channels 106 areconfigured so that the CM maintains thermal contact with heat sink 102for a relatively long transit time in flowing through the flow channelto shuttle 28. During the transit time, the CM absorbs heat from heatsink 102 so that upon exit from the heat sink its temperature issubstantially that of the desired temperature at which the heat sink ismaintained by heating elements 104. In accordance with an embodiment ofthe invention, the length of at least one flow channel 106 issufficiently long and its internal width sufficiently small so that forsubstantially all rates at which printing heads in shuttle 28 dispenseCM, the CM is heated by flow-heater 100 to the temperature at which heatsink 102 is maintained. By way of numerical example, each channel 106may be about 150 mm long and have a cross section diameter of about 4mm.

Alternatively or additionally, when the flow channel is not sufficientlylong, the temperature of the heat sink is adjusted based on the flowrate, with a higher temperature being provided to compensate for thefact that heat transfer per unit volume of the fluid is reduced when theflow increases. Thus for relatively low flow rates the heat sinks may beheated to the same temperature as the CM in the reservoir, or eventurned off. For higher flow rates, the heat sink is heated to a highertemperature, such that even in the limited time available in which theCM flows through the flow-heater, the CM reaches a temperature at ornear that of the CM in the reservoir. In general, the temperature of theCM leaving the flow heater should be high enough (preferably as high asrequired at the outlet of the fluid reservoir) so that the reservoirheater can heat it to a proper operating temperature before dispensingit. This principle is preferably followed for each of the embodimentsshown.

If flow-heater 100 is used to control dispensing temperature for asingle CM, or different CMs having a same dispensing temperature,controller 26 controls heating elements 104 to maintain the temperatureof heat sink 102 at the dispensing temperature. After the CM exitsflow-heater 100 it enters a reservoir or reservoirs (not shown) inprinting head block 50 which optionally have conventional heaters (notshown) that function to maintain the CM dispensing temperature. Ifflow-heater 100 is used to control temperature of different CMs havingdifferent dispensing temperatures (for example, SM may require adispensing temperature of about 60° C. and BM a temperature of about 70°C.), controller 26 controls heaters 104 to maintain the temperature ofheat sink 102 at about the lowest of the dispensing temperatures. Theconventional heaters (of the reservoirs) function to raise thetemperatures of the CM or CMs having the higher dispensing temperaturesto their respective dispensing temperatures.

FIG. 1A shows preheater 100 and head block 50; both are separate fromeach other and therefore connected by two pipes, which enable thematerial flow between said two components. The preheater and the blockmay, however, being firmly connected, for example by having the twocomponents being manufactured as a cast of the same metal body, and theheat transfer between the two components may be large respectively. Inthat case, a single heating and control device, which heats up thecommon optionally metal body to a common temperature, may serve for bothcomponents.

In some embodiments of the invention, a CM reservoir in an RPM printinghead block comprises internal components that function to provideimproved control of dispensing temperature of a CM so that the CM issupplied to printing heads at a desired dispensing temperaturesubstantially independent of a rate at which the CM is supplied from thereservoir. The components cooperate to provide a relatively long flowpath inside the CM reservoir for CM flowing through the reservoir. Theflow path maintains the CM in thermal contact with a heating elementmaintained at the desired dispensing temperature for a sufficient timeso that the CM is heated to the desired dispensing temperaturesubstantially independent of a flow rate of the CM through thereservoir.

FIG. 1C schematically shows a CM reservoir 400 comprised in a printinghead block (not shown), in accordance with an embodiment of theinvention. CM flows into reservoir 400 at an inlet 402. Upon enteringreservoir 400, the CM encounters a series of baffles 404 mounted betweena wall 406 of the reservoir and a plate 408 that constrains the CM toflow between the wall and the plate along the baffles as indicted byblock arrows 410 to enter a “holding volume” 412 of the reservoir.

At least one heating element maintained at a desired dispensingtemperature of the CM is mounted, optionally inside reservoir 400, towall 406 and/or plate 408. By way of example, a planar heating element414 is mounted to wall 406. The “baffle path” created by baffles 404along which the CM flows in order to enter holding volume 412 ofreservoir 400 maintains CM that enters inlet 402 in thermal contact withheating element 406 long enough so that by a time it enters holdingvolume 412, the CM is heated to its desired dispensing temperaturesubstantially independent of a rate at which the CM exits the holdingvolume via an outlet 416.

In some embodiments of the invention outlet 416 is placed closer toplate 408. In others is placed between heater 414 and plate 408 andplate 408 is longer and connected to the bottom (base) plate ofreservoir 400, in that case an aperture is formed in plate 408 near thebottom plate and far from outlet 416, to connect the main reservoir tothe volume between heater 414 and plate 408. In this embodiment, theheating of the material is carried out mainly in the volume betweenheater 414 and plate 408 and the main function of the main reservoir isto reduce the effect of changes in height of the material as material isadded via entrance 402 and removed via outlet 416. The present inventorshave found that changes in height of the material can change the flowrates of material from outlet 416. Using the larger reservoir as ballastreduces any such effects.

Optionally plate 408 is a good conductor of heat so that heat fromheating element 414 flows into holding volume 412 to maintain CM in theholding volume at the desired dispensing temperature. In an embodimentof the invention, plate 408 is constructed so that a region of the plateclose to inlet 402 is a poor conductor of heat or is insulated toprevent heat transfer through the upper region and the rest of the plateis a good heat conductor. The heat insulating portion of plate 408prevents relatively cold CM just entering inlet 402 from absorbing heatfrom CM already in holding volume 412 while the portion of plate 408that is a good conductor enables CM newly heated by heating element 414to “share” heat from heating element 414 with CM in the holding volume.Optionally, plate 408 is also heated.

Configurations of baffles and baffle paths and heating elementsdifferent from that shown in FIG. 1C may be advantageous in the practiceof the invention. FIG. 1D schematically shows another CM reservoir 420comprising a baffle path in accordance with an embodiment of theinvention.

Reservoir 420 comprises a least one inlet 422 through which CM entersthe reservoir, a least one outlet 424 from which CM is supplied toprinting heads in an RPA such as RPA 20 (FIG. 1) and optionally parallelbaffle plates 426 that are tilted downward towards a heating element428. Each baffle plate 426 is connected to three walls 430 of thereservoir along three of the edges of the baffle plate. A fourth edge432 of each baffle plate is spaced from heating element 428. Reservoir420 is fillable with CM through inlet 432 so that the CM fills thereservoir and the spaces between baffle plates 426. To enable CM to fillthe spaces between baffle plates 426, the baffle plates are optionallyformed with relatively small apertures 434 that allow air to escape thespaces during filling of the reservoir. Due to the limited contact of CMwith heater 428 it may be advantageous to heat plates 426.

CM flows out of reservoir 420 to printing heads via outlet 424. Whenflowing out of the reservoir, substantially all CM flows alongdirections indicated by block arrows 436 and maintains contact withheating element 428 until it reaches the bottom of the reservoir andexits through outlet 424. Because of the relatively small size of airapertures 434 relatively little of the CM flows out of the reservoir viathe apertures 434 and avoids contacting heating element 428. Contact ofthe CM with heating element 428 operates to keep CM flowing out ofreservoir 420 at a desired dispensing temperature. The heating elementis maintained at a suitable temperature so that for an operating rangeof CM flow rates out of reservoir 420, CM flowing out of the reservoircontacts the heating element for a sufficient time so that itstemperature is substantially equalized to that of the desired dispensingtemperature. Assuming that the lower section of the reservoir isthermally isolated from the surrounding, the heated liquid is thenmaintained at the operating temperature in the lower section ofreservoir 420.

When objects 22 are completed, they are removed from production platform24 by separating foundation 35 from the construction platform. Asindicated above, in the prior art, the objects were removed by using ascraper to separate object 22 and foundation 35 from the constructionplatform. Since the scraping of the foundation from the constructionplatform tends to cause bending of the object, especially when thescraper is not parallel to platform 24, the foundation had to be madethick enough to avoid excess stress on object 22. Typically, the priorart foundation was in the range of 2-5 mm thick.

In accordance with an embodiment of the invention, foundation 35 isseparated from construction platform 24 using a spatula shovel 115schematically shown in FIG. 2, which figure also schematically shows theplatform with objects 22 completed.

Spatula shovel 115 comprises a relatively thin flat plate 117 having alength and width substantially the same as that of the top surface ofconstruction platform 24 or at least as wide as foundation 35, arelatively sharp front “knife edge” 119 and optionally upturned side andback edges 121. Optionally, spatula shovel 115 comprises handles 122.Plate 117 is formed thinner than the scraping blades of conventionalscrapers and is advantageously formed from sheet metal such as steel.Preferably, plate 117 has a thickness in a range from about a few tenthsof a mm to about 0.5 mm. Upturned edges 121 serve to provide structuralstrength for the plate. “Knife edge” 119 is optionally manufactured sothat the upper side of the edge is a slope but the lower edge is flat.When the spatula is moved into the foundation, the friction of thefoundation with the edge forces the edge in the direction ofconstruction platform 24 because of said slope of the edge. Thisprevents collision of the knife with the object above the foundationeven when the spatula is parallel or close to being parallel to theplatform.

As indicated above, the prior art provides a relatively thick platformso that the object could be removed using a knife or small spatula orthe like such that the object would be protected from stress duringremoval. In accordance with various embodiments of the presentinvention, a relatively wide spatula, having a width at least as largeas the object that is to be removed is used. Optionally, the spatula isthin (or at least has a thin leading edge) so that the spatula can beslid under the (thinner) platform without over stressing the object. Asopposed to the prior art method in which the object is lifted as theknife or spatula is forced under the platform, in accordance with anembodiment of the invention, the spatula is kept substantially parallelto the production platform during removal of the object.

To remove objects 22 from construction platform 24, front edge 119 ispressed to foundation 35 in a direction indicated by a block arrow 123with the sharp edge oriented so that it substantially contacts thesurface of the construction platform. The force with which front edge119 is pressed to foundation 35 forces the front edge between thefoundation and construction platform 24 to peel and/or scrape and liftthe foundation off the construction platform 24, and with thefoundation, objects 22, without substantially bending the foundation.After separating foundation 35 and objects 22 from construction platform24, the foundation and objects rest on flat plate 117 and spatula shovel115 functions as a carrying tray for the objects.

In order to moderate probability of damage to finished objects resultingfrom using prior art scrapers to separate a foundation on which thefinished objects are produced from an RPA construction platform,foundations are typically formed about 2 mm thick. The inventors havefound that using a “thin” spatula shovel similar to spatula shovel 115,to remove objects from a construction platform, in accordance with anembodiment of the invention, enables foundations such as foundation 35and the objects formed thereon to be removed relatively easily andsafely from the construction platform. As a result, foundations may bemade thinner, optionally less than 1 mm thick and as thin as about 0.3mm, leading to savings in production time and cost.

For example, assume construction platform 26 is about 40 cm wide andabout 50 cm long. If foundation 35 is about 2 mm thick and has lengthand width about the same as that of construction platform 26, thefoundation has a volume of about 400 cm². Support material from whichfoundation 35 is optionally formed typically has a specific gravity ofabout 1 g/cm² and may cost about $150/kg, resulting in a material costfor the foundation of about $60. Assuming that a construction layerformed by RPA 20 is about 16 microns thick and takes about 25 seconds toprint, it takes about 50 minutes to produce a conventional foundation35. Foundation 35 formed with the expectation that it will be separatedfrom construction platform 24 using a spatula shovel in accordance withan embodiment of the invention may be made, for example, at a thicknessof 0.5 mm noted above and therefore at one fourth the material cost andone fourth the production time of a conventional foundation. It is notedthat whereas FIG. 2 schematically shows foundation 35 as a single largefoundation on which all of objects 22 are built, in some embodiments ofthe invention each object 22 is built on its own relatively small“island foundation” formed on construction platform 24. Building objectson relatively small island foundations can result in saving foundationmaterial and can make removing objects after they are built easier.

Quality of objects such as objects 22 produced by RPA 20 depends, interalia, upon how accurately CM is dispensed at locations at which it isdesired to dispense the CM. Generally, a printing head block, such asprinting head block 50 comprises a plurality of printing heads, eachhaving a linear array of equally spaced output orifices through which CMis dispensed. Accurate orientation and positioning of the printing headsrelative to each other and to scanning and displacement directions ofshuttle 28 are advantageous for providing accurate dispensing of CM.

FIG. 3 schematically shows printing head block 50 shown from itsunderside and a plurality of printing heads 130 mounted to the printinghead block. Orientation of printing heads 130 relative to FIG. 1A isindicated by the relative orientation of coordinate system 21, used toprovide orientation in FIG. 1, in FIG. 3.

Each printing head 130 is formed having an array of output orifices 132accurately aligned along a straight line 134. Straight lines 134,hereafter referred to as “printing head axes”, along which orifices in aprinting head 130 are aligned, are shown for some of the printing heads.A printing head block and printing heads similar to printing head block50 and printing heads 130 are described in PCT application WO2004/096527, the disclosure of which is incorporated herein byreference.

For convenience and simplicity of operation of an RPM similar to RPM 20,(FIG. 1), to provide accurate and convenient dispensing of CM, printinghead block 50 and its print heads 130 are advantageously mounted in theRPM so that axes 134 are accurately parallel to each other and,optionally, accurately perpendicular to the scan axis which is assumedto be the x-axis. The printing head block is moved along a displacementaxis, which is advantageously parallel to the printing head axes 134 andoptionally perpendicular to the x-axis, i.e. parallel to the y-axis ofcoordinate system 21.

Distance, referred to as “x-offset”, between adjacent printing head axes134 is represented by Δx and distance, referred to as “inter-orificey-offset”, between output orifices 132 in a same printing head isrepresented by Δy. A first orifice 132 in a printing head 130 is anorifice having a smallest y-coordinate and a last orifice in theprinting head is an orifice having a largest y-coordinate. First andlast orifices 132 for some printing heads 130 are indicated by letters“F” and “L” respectively. Different printing heads 130 are optionallymounted to printing head block 50 displaced one from the other along they-axis by a “printing head” y-offset ΔY_(ph) so that during a scan,homologous printing orifices 132 in different printing heads 130 printalong different parallel lines on construction platform 24 (FIG. 1A).Assume that N printing heads 130 mounted to printing head block 50 areused to simultaneously print a same CM. Then ΔY_(ph)=Δy/N.

The x-offset, Δx, advantageously has a same normative value for any twoadjacent printing heads 130. Let an integer index “n” individualizeprinting heads 130 and have a value that differs by one for adjacentprinting heads 130 and an initial value 0 for a first printing head 130,which is, optionally, a printing head having a smallest x-coordinate.Then, if printing head axis 134 of the first printing head 130 has anx-coordinate x_(o), the printing head axis of an n-th printing headshould ideally have an x-coordinate equal to x_(n)=x_(o)+nΔx.

Assume further that each printing head 130 has “M” output orifices 132,and a same normative inter-orifice y-offset Δy between adjacent outputorifices 132 in the printing head. A distance MΔy is referred to as a“shuttle Y-span (SYS)”. A distance between the first and last orifice132 of a printing head 130 is equal to (SYS−Δy)=(M−1) Δy and isindicated in FIG. 3. If shuttle 28 is properly aligned, a displacementof the shuttle along the y-axis by a distance equal to SYS should resultin the first output orifice in the printing head having a y-coordinategreater by Δy than the y-coordinate of the last orifice 132 in theprinting head prior to the y-axis displacement.

In accordance with an embodiment of the invention, an RPA such as RPA 20is calibrated to determine if print heads 130 are properly aligned andif not, to undertake procedures for correcting misalignment.

To calibrate RPA 20 so that its print heads 130 are perpendicular to thescan direction, i.e. the x-axis, it is assumed that printing head axes134 are mounted in print-head block 50 so that their axes 134 areaccurately parallel to each other. Any method known in the art, forexample a method described in PCT application WO 2004/096527 referencedabove, may be used to mount print heads 130 in printing head block 50 sothat their axes 134 are accurately parallel. Rotation of printing headaxes 130 away from the perpendicular to the x-axis is assumed to resultfrom an unwanted rotation in the position of printing block 50 relativeto the x-axis. Or for example, as a result of all the printing head axes134 being skewed relative to the printing head block.

Calibration to align printing head axes 134 perpendicular to the x-axisis performed by printing a first line parallel to the x-axis with an“i-th” orifice in a k-th printing head 130 and a second line parallel tothe x-axis with a j-th orifice in an 1-th printing head 130. A lineparallel to the x-axis may be printed with the i-th orifice by suitablydepositing droplets of CM (BM or SM) from the orifice during a scan passof shuttle 28. If printing heads 130 are properly aligned with theirrespective axes 134 accurately perpendicular to the x-axis the twoprinted lines should be separated along the y-axis by a nominal distance“D_(y)”=|(i−j)Δy+(1−k)ΔY_(ph)|. If on the other hand, the separationdeviates from the nominal separation, printing head axes 134 are rotatedaway from the perpendicular to the x-axis. Let the separation betweenthe printed lines be less than (the printed separation can only be lessthan the nominal distance) the nominal distance by an amount δ_(y). Thenan angle by which the printing head axes are rotated away from theperpendicular is equal to about √{square root over (2)}δ_(y)/D_(y). Ifδ_(y) is not zero, the orientation of the printing head block isadjusted to reduce or substantially remove the deviation. It is notedthat the deviation δ_(y) does not indicate whether the angle by whichthe printing head axes is rotated away from the perpendicular isclockwise or counterclockwise. Determination of the sign of the rotationthat gives rise to the deviation may require a trial and errorprocedure.

Assuming printing head block 50 is oriented so that printing head axes134 are substantially perpendicular to the x-axis, RPA 20 (FIG. 1A) iscalibrated to determine and correct deviation from a normative x-offsetspacing Δx between printing head axes 134.

To determine if the x-offset between any two, first and second printingheads 130 in RPA 20 is correct, in accordance with an embodiment of theinvention, RPA 20 is controlled to print a first straight line parallelto the y-axis with the first printing head 130. A line parallel to they-axis may be printed by suitably depositing droplets of CM from aplurality of orifices 132 in the printing head. Under the assumptionthat the x-offset between the printing heads is correct and equal to thenormative x-offset spacing Δx, RPA 20 is then controlled to attempt toprint a second straight line collinear with the first line using thesecond printing head 130. Optionally, the y-axis lines are printed bythe first and second print heads during a same scan of shuttle 28 in thex-direction. If the assumption that the x-offset between the printingheads is equal to the normative x-offset spacing Δx is correct, thesecond line will be collinear with the first line. On the other hand, ifthe assumption is wrong, the first and second printed lines will bedisplaced one from the other.

If the lines are displaced from each other, optionally, the printingheads 130 are mechanically realigned to correct for the incorrectx-offset spacing. Alternatively or additionally, in accordance with anembodiment of the invention, the incorrect x-offset is compensated forby correcting the relative timing of signals that trigger orifices 132of the printing heads to dispense CM. The trigger signals are generallyprovided by suitable encoders and/or step motors that trigger orifices132 of the printing heads to dispense CM responsive to position ofshuttle 28 and assumed x-offset spacings between printing heads 132. Tocorrect x-offset errors, positions of shuttle 28 at which orifices 132in printing heads are triggered to dispense CM are adjusted so that theydispense CM at their nominal spacings.

For example, assume that an x-offset error in alignment between an i-thand a j-th printing head 130 equal to e_(x) is to be corrected. Were theprinting heads perfectly aligned in the x-direction, for the printingheads to print collinear lines during a same scan of shuttle 28, thej-th printing head would be triggered to print at a position of shuttle28 along the x-axis that differs from a position of the shuttle at whichthe i-th printing head prints a line by a “normative displacementdistance” equal to (j−i) Δx. To compensate for the x-offset error e_(x),the encoders and/or step motors and/or processing of signals theyprovide are adjusted so that the j-th printing head is triggered at aposition of shuttle 18 along the scan direction that differs from aposition at which the i-th printing head is triggered by a distance,hereinafter a “corrected displacement distance”, equal to[(j−i)Δx+e_(x)] rather than the normative displacement distance[(j−i)Δx].

In an embodiment of the invention, to calibrate the x-offset spacings ofprinting heads 130 in printing block head 50, and determine correcteddisplacement distances for the printing heads, all the heads arecalibrated against a same “standard” head 130. Optionally, the standardhead against which the other heads are calibrated is a first head, atone end of the printing head block. For each of the other heads, thecontroller controls shuttle 28 to print with the standard printing heada set of standard lines parallel to the y-axis. Optionally, the linesare equally spaced. Optionally, the controller controls the shuttle toprint a different standard line set for each of the other heads to becalibrated. FIG. 4A schematically shows a set 140 of standard lines 141printed by the standard printing head to calibrate the x-offset ofanother, second printing head. Optionally, lines 141 are printed on atransparent substrate 144 to facilitate observing the lines under amicroscope.

For each standard line 141 in standard line set 140, shuttle 28 iscontrolled to print (with the second printing head) a different“calibration” line shown as a dashed line 142 in FIG. 4A parallel to they-axis. Each calibration line 142 is printed at a different“calibration” displacement distance relative to a position of shuttle 28at which its corresponding standard line 141 is printed. Optionally, oneof the calibration displacement distances is equal to a normativedisplacement distance (i.e. (j−i)Δx where the i-th printing head is thestandard printing head and the j-th printing head is the second printinghead) for the standard and second printing heads and each of the othercalibration distances differs from the normative displacement distanceby a different, small perturbation distance. A corrected calibrationdistance for the second printing head is determined to be a calibrationdisplacement distance for which a calibration line 142 closest to beingcollinear with its corresponding standard line 141 is printed by thesecond printing head 130. In FIG. 4A a calibration line 142 labeled withthe letter C is closest to being collinear with its correspondingstandard line 141 and the calibration displacement distance at which itis printed is determined to be the corrected displacement distance forthe standard and second printing heads 130. It is noted that whereas inFIG. 4A the standard and second printing heads are controlled to printsets of lines, an x-offset spacing calibration, in accordance with anembodiment of the invention can be performed by controlling the printingheads to print “dots”, i.e. standard and calibration dots, instead oflines.

In accordance with an embodiment of the invention, it may be useful tocalibrate RPA 20 so that the displacement direction of print heads 130is parallel to printing head axes 134. Two lines are printed at a givenx-coordinate with a given printing head 130. The first line is printedon an appropriate substrate by depositing droplets of a CM from a sameoutput orifice 132 as the given printing head 130 is moved along thedisplacement direction (i.e. optionally the y-direction). This sameorifice is preferably one of the lower orifices of the head. Afterdisplacing the head substantially SYS along Y, at the same givenx-coordinate, a second line is printed with a different orifice 132 ofthe same printing head by depositing droplets of CM from the orifice asthe given printing head 130 is moved along the displacement direction.This same orifice is preferably one of the upper orifices of the head.If the lines are collinear, the displacement direction is parallel tothe printing head axes 134 and, optionally, the y-axis. If not, thedisplacement direction is not parallel to printing head axes 134.

Alternatively, each line is printed by activating all of the orifices ata given x-position. This will result in a line of dots substantiallyalong line 151 of Fig. Then the shuttle is moved in the y-direction bysubstantially SYS. Again, all the orifices at the same x-position areactivated. This produces a line of dots along line 152. Alternativelyonly few orifices at the lower portion of the head are activated in thefirst time, and few orifices at the upper portion of the head areactivated in the second time.

FIG. 4B schematically shows first and second lines 151 and 152 printedusing the second method. Line 151 is printed using all of the orifices132 in a same printing head 130. Then the print head is displaced alongthe y-axis a distance of approximately SYS. Another line 152 is thenprinted using all the orifices. If the displacement direction isparallel to printing head axis 134 of the printing head, lines 151 and152 are collinear. If on the other hand the displacement direction wererotated away from the parallel to the printing head axis 134, lines 151and 152 would not be collinear.

FIG. 4C schematically shows first and second lines 151 and 152 accordingto the second method, assuming that the displacement direction isrotated away from the parallel to the printing head axis 134 by an angleα. The lower end 153 and upper end 154 of lines 151 and 152 respectivelyare separated by a distance D_(x) along the x-axis, and the angle α isapproximately equal to D_(x)/SYS assuming a small angle approximation.

For a situation in which the displacement direction is not parallel tothe axis 134 of a printing head 130, optionally the relative orientationof the displacement direction vs. head axis 134 is correctedmechanically to reduce the unwanted rotation angle α to substantiallyzero. Additionally or alternatively, in accordance with an embodiment ofthe invention, the rotation angle α is compensated for by adjustingrelative times at which output orifices 132 in the printing head arecontrolled to dispense CM.

Though not explicitly stated, it is noted that the above calibrationprocedure assumes that once printing head axes 134 are alignedperpendicular to the x-axis, aligning the printing head axes parallel tothe y-axis, if they are not already so, does not affect theirorientation relative to the x-axis. This may depend on how shuttle 28 ismounted in RPA 20. And an order in which the alignment proceduresdescribed above are performed may have to take into account how shuttle28 is coupled to RPA 20.

For example, if the shuttle is constrained to translate along a rail (an“x-rail”, which defines a scan direction and thereby an x-axis), oncethe printing head axes 134 are aligned perpendicular to the x-axis theyare substantially fixed in an orientation perpendicular to the x-rail.Printing head axes 134 may then be aligned parallel to a y-axis, i.e. adisplacement direction, by orienting the x-rail and y-axis,perpendicular to each other without affecting orientation of theprinting head axes relative to the x-axis. (Though, as a result, thex-axis, or the y-axis might change orientation relative to features ofRPA 20).

On the other hand, assume shuttle 28 is constrained to translate along arail, a “y-rail” that defines a y-axis and a displacement direction. Ifthe printing head axes 134 were aligned first perpendicular to a scandirection defined by translation of the y-rail along an x-axis, were theprinting head axes not subsequently parallel to the y-axis, aligning theprinting head axes parallel to the y-axis would rotate the printing headaxes away from the perpendicular to the x-axis. To align printing headaxes 134 in this case, the printing head axes should first be alignedparallel to the y-axis.

In order to calibrate RPA 20 so that controller 26 moves shuttle 28 a“correct” distance parallel to the y-axis when it is required to print aconstruction layer having a “width” along the y-axis greater than theshuttle y-span SYS, the shuttle is controlled to print a line parallelto the x-axis with, optionally, a last orifice 132 (i.e. an orificelabeled L) in a printing head 130. The line is optionally printed on atransparency. Controller 26 then displaces shuttle 28 along the y-axisby a distance equal to SYS and prints another line on the transparency,this time, optionally, using a first orifice 132 (i.e. an orificelabeled F), in the print head. A distance between the lines, optionallychecked using a microscope, should be equal to the normative y-offset,Δy. If the distance differs from Δy, controller 26 is suitably adjustedto correct for the difference. Adjustment of the controller is generallyaccomplished by adjusting the controller software. It is noted that inthe above calibration procedure, orifices 132 other than a first and alast orifice 132 may be used to print “calibration lines”. Depending onthe orifices used, a distance between calibration lines may be differentthan ΔY.

The above procedure for calibrating RPA 20 so that displacement alongthe y-axis by a distance SYS results in a displacement of print heads130 that positions them properly to print construction layers wider thanSYS assumes that the distance SYS is a given, equal to MΔy. However,different objects produced by an RPA such as RPA 20 are often producedat different operating temperatures that result in changes in thedimensions of printing heads 130 from object to object. In additionduring cooling of printed CM material the material generally shrinks andthe cooled dimensions of a construction layer can be different from theprinted dimensions of the layers. To account for these factors, aconstruction layer having width W is printed at an operating temperatureat which an object is to be produced by RPA 20 and the material in theconstruction layer allowed to cool and measured after cooling. Assumingthat the cooled layer has a width W*, displacements along the x andy-axis are adjusted by a scale factor SF=W/W* to compensate for theeffects of cooling and operating temperature. Optionally the width W isa largest printable width of RPA 20.

In all (or most of) the calibration methods described above, the linesand dots between which a distance is to be measured, are designed to bevery close to each other (less than 5 mm apart, preferably less than 1mm). This is done in order to minimize errors that may be brought aboutby expansion or contraction of the substance on which the line or dotsare printed (often a plastic transparency, and minimizing accuracyerrors of the measurement tool (a microscope or comparator), andenabling the use of simple optical tools having small veining fields(like loops).

Cost of an object produced by an RPA depends upon amounts of materialused in producing the object and its production time. Whereas, ingeneral there is little control over an amount of BM required to producean object because the amount is determined by the object's volume, anamount of SM required to produce the object depends, in general, uponits orientation on the RPAs production platform. Production time is afunction inter alia of a number of scan passes, i.e. passes along thescanning x-direction that the shuttle makes in order to produce theobject, and the lengths of the scan passes. The number and lengths ofthe scan passes are functions of orientation and relative position ofthe objects on the construction platform.

In order to utilize a minimum amount of SM, the orientation of theobject is important. This can be understood most easily if it is desiredto produce a hollow pyramid, closed on all sides. If the pyramid isproduced point down, then support material has to be provided to supportall of the side walls and also the wall that is at the top of thepyramid as produced. However, if one of the walls is at the bottom ofthe object as produced, only the side walls need to be supported. Thepoint down construction requires roughly twice the support material asdoes the point up construction. Of course, it would be immediatelyevident, to a person of the art in this situation, that it makes muchmore sense to construct the object point up. In other situations it isless clear which configuration is best. The following procedure is used,according to some embodiments of the invention, to determine the optimumconfiguration.

In accordance with an embodiment of the invention, a build configurationthat requires a relatively small amount of support material duringproduction of an object is determined in accordance with a method 200whose acts are shown in FIGS. 5A and 5B. Method 200 is illustrated inFIGS. 6A-6M for an exemplary construction object 190. FIGS. 6A-6Mschematically show “virtual images” of the exemplary construction objectand construction platform 24 relative to coordinate system 21, shown inFIG. 1A. For each of FIGS. 6A-6M, the corresponding act or acts inmethod 200 is given in parentheses next to the figure number.

For construction data that defines object 190 and a given initialorientation of the object is provided. At 202 (FIG. 5A) the surfacevectors 192 (FIG. 6A) are defined for the construction object.Optionally, surface vectors 192 are defined conventionally. The surfacevectors are perpendicular to the surfaces of construction object 190,point outward from its volume and each vector has a size proportional tothe area of its corresponding surface. At 204, for each group of vectors192 that are substantially parallel to a same direction, a “groupvector” is defined having a magnitude equal to a sum of the absolutevalues of all the substantially parallel vectors and a direction that isthe same as a direction to which the vectors are substantially parallel.Optionally the surface vectors in a group of vectors that is used todefine a group vector are characterized in that an angle between any twovectors in the group is less than a predetermined maximum angle.Optionally the maximum angle is less or equal to about 1°. Optionally,the maximum angle is less than or equal to about 5°.

At 206 (FIG. 5A), construction object 190 is oriented so that a largestgroup vector of the group vectors defined for object 190 is parallel oranti-parallel to the stacking direction i.e. the z-axis. By way ofexample, FIG. 6B shows object 190 with two group vectors, vectors 194and 196, determined for the object. Of the group vectors, vector 194 isthe largest group vector and object 190 is oriented by method 200 withgroup vector 194 parallel to the z-axis.

Optionally, at 220, an initial orientation is determined for object 190by rotating the object about an axis parallel to the z-axis so that agroup vector, by way of example group vector 196, having a maximumcomponent perpendicular to the z-axis, is parallel to the y-z plane.FIG. 6C shows object 190 oriented by method 200 so that group vector 196is parallel to the y-z plane. For the initial orientation established at220, object x′, y′, and z′-axes, shown in FIG. 6D, that are parallelrespectively to the x, y and z-axes are defined for object 190 at 221.Once determined, the object x′, y′, and z′-axes are fixed relative toobject 190.

At 222 object 190 is optionally rotated to each orientation for which anobject x′, y′ or z′-axis is parallel or anti-parallel to the z-axis. Foreach of the orientations, an amount of SM required to produce object 190is determined. Optionally, a volume V_(SM) of SM required for a givenorientation of object 190 is determined by integrating the height of theobject in the given orientation over the x-y plane and subtracting thevolume of the object. FIGS. 6D, 6E and 6F show orientations inaccordance with an embodiment of the invention for which one of theobject x′, y′ or z′-axes is parallel to the z-axis. A shaded area, orareas, 198 in each of the figures schematically indicates where SMmaterial is required to be deposited during construction of object 190.An orientation for which the determined amount of SM is a minimum is,optionally determined at 223, to be an “SM advantageous orientation”. Ofthe orientations shown in FIGS. 6D-6F, FIG. 6F shows an SM advantageousorientation, for which SM is a minimum.

Optionally, at 223 object 190 is also rotated about an axis parallel tothe z-axis so that a group vector of the object having a maximumcomponent perpendicular to the z-axis, is parallel to the y-z plane.While this latter rotation does not change an amount of SM required toproduce object 190, it will in general reduce a distance along they-axis that a shuttle, e.g. shuttle 50 (FIG. 1) has to be displaced toproduce the object. Reducing the y-axis displacement in general resultsin a shorter production time for the object. Methods of reducingproduction times in accordance with embodiments of the invention arediscussed in greater detail below.

Optionally, in accordance with an embodiment of the invention, aninitial orientation for object 190 is also, or alternatively, determinedfrom the orientation determined at 206 differently from the way aninitial orientation is determined at 220. At 210, method 200 rotatesobject 190 about an axis parallel to the z-axis until a volume of anenvelope box 201 comprised of x-y, x-z and y-z oriented planes thatcontains object 190 is minimum. An orientation of object 190 for whichenvelope box 201 has a minimum volume, and which is therefore, inaccordance with an embodiment of the invention, an initial orientation,is shown in FIG. 6H. It is noted that the initial orientation determinedat 210 (FIG. 6H) is by way of example, different from that determined at220 (FIG. 6C).

Similarly to the case for the initial orientation determined at 220, at211 object axes x′, y′ and z′ are determined for the initial orientationdetermined at 210 (FIG. 6H). Thereafter, at 212, object 190 isoptionally rotated into each of the orientations for which an x′, y′ orz′ object axes is parallel or anti-parallel to the z-axis and for eachof the orientations an amount of SM required to produce the object isdetermined. FIGS. 6J, 6K and 6L, show some orientations for which method200 determines an SM quantity at 212. In the figures regions thatrequire deposition of SM material are indicated by shaded regions 198.

Optionally, at 213, method 200 determines for which of the orientationsdetermined in step 212 the SM quantity is minimum. The orientation forwhich the amount of SM is a minimum, and which is therefore an SMadvantageous orientation, is the orientation shown in FIG. 6L. It isnoted that in the specific example shown in FIGS. 6A-6L the differentinitial orientations determined at 210 and 220 lead to different SMadvantageous orientations and that the SM advantageous orientation shownin FIG. 6L is different from that shown in FIG. 6F. Optionally, as at223, object 190 is rotated at 213 about an axis parallel to the z-axisso that a group vector of object 190 having a maximum componentperpendicular to the z-axis, is parallel to the y-z plane.

Additionally or alternatively, method 200 optionally determines an SMadvantageous orientation at 230. At 230 the method determines that an SMadvantageous orientation is that for which a difference in az-coordinate of the center of mass of object 190 and that of productionplatform 24 is minimum, i.e. an orientation for which the center of massof the object is lowest relative to production platform 24. FIG. 6Mschematically shows a location 199 of the center of mass of object 190and an SM advantageous orientation of object 190 for which the center ofmass is closest to platform 24. By way of example, for object 190 thelowest center of mass SM advantageous orientation shown in FIG. 6M isthe same as the SM advantageous orientation determined at 223 and shownin FIG. 6F. At 231, method 200 optionally determines an amount of SMrequired to produce object 190 in the lowest center of mass SMadvantageous orientation shown in FIG. 6M. And as at 223, object 190 isrotated about an axis parallel to the z-axis at 231 so that a groupvector of object 190 having a maximum component perpendicular to thez-axis, is parallel to the yz-plane.

At 240, algorithm 200 optionally compares the quantities of SM requiredto produce object 190 for each of the SM advantageous orientations(FIGS. 6F, 6L, 6M,) as determined by the algorithm and determines abuild configuration for the object to be that SM advantageousorientation for which the SM quantity is a minimum. By way of example,for object 190, the build configuration determined by algorithm 200 isthat shown in FIGS. 6F and 6M.

In some embodiments of the invention, following a determination of groupvectors for an object, for example as noted at 204 (FIG. 5A), for eachgroup vector, a volume V_(SM) of SM for the object is determined for thegroup vector parallel to the z-axis and anti-parallel to the z-axis.Optionally, for each orientation of the group vector, V_(SM) isdetermined as noted above. The group vector and its orientation alongthe z-axis for which V_(SM) is a minimum is determined to be anadvantageous SM orientation.

In some embodiments of the invention, a plurality of axes is defined foran object that intersect at a same internal point of the object. Theobject is oriented at a plurality of different orientations so thatpoints at which the axes intersect the surface of a sphere having itscenter at the internal point are homogeneously distributed over thesurface of the sphere. For each orientation, a volume V_(SM) of SMrequired for the orientation is determined. An orientation that has aminimum V_(SM) is determined to be an advantageous SM orientation.

Whereas method 200 determines a build configuration of an object thatprovides for production of the object using a relatively small amount ofSM, and optionally determines an advantageous orientation for productionof the object to reduce production time, the method does not indicatehow a plurality of objects may be advantageously arranged for productionon a construction platform. For situations for which a plurality ofobjects are to be produced at a same time, the relative positions of theobjects on construction platform 24 as well as their orientations, canaffect, inter alia, production time of the objects.

A total production scan length for production of a construction objector a plurality of construction objects that are to be simultaneouslyproduced by an RPA is equal to the sum of scan lengths of the scanpasses that are required to produce the object or objects. Let anenvelope volume be a volume that just envelops the construction objector construction objects. The size and/or orientation of the envelopevolume are determined by the orientation and relative position onplatform 24 of the object or objects. Often, to an extent that theenvelope volume is smaller, the total production scan length is smallerand as a result, the production cost of the object or objects smaller.In addition, for a given total production scan length, to an extent thata number of production scan passes required to produce an object orobjects is smaller, the production time, and generally production cost,is smaller. A dimension of the enveloping volume along the displacementdirection is proportional to the number of production scan passes.Therefore, generally, to an extent that a dimension of the envelopevolume is smaller along the displacement, the production time and costare smaller.

As discussed above, production time of a layer is a function inter aliaof an overall number of scan passes required to produce the object, andthe lengths of the scan passes. The number of scan passes has a greaterimpact on production time than the length of scan passes. This is due tothe fact that the head travels a substantial length in X direction priorand after actual printing on the tray, as well as theacceleration/deceleration and Y indexing between scan passes requiredfor each pass. We, therefore, suggest a method that primarily minimizesthe number of scan passes. The method would not necessarily lead to thebest solution for every case, but would enable reasonable placement inthe majority of tray designs.

Optionally, to determine a build configuration for a plurality ofobjects for production on production platform 24, the objects areordered in descending order of height (i.e. the dimension in Zdirection). The objects are then positioned one after the other onproduction platform 24 in accordance with their order.

Alternatively, the objects are ordered according to their “Y×Z” figure.“Y×Z” figure is the sum of Y lengths modulus SYS (shuttle Y-span) of alllayers along the object's projection on plane Y×Z (See FIGS. 7A and 7B).Y lengths modulus SYS is a measure of the amount of scan passes that arerequired to dispense the layer.

In some embodiments of the invention, to orient and position a pluralityof objects for production on platform 24, each of the objects isoptionally oriented so that its projection on the y-z plane isrelatively small or substantially a minimum, and the objects are orderedin descending order by height or by “Y×Z” figure. Optionally theorientation of each object is determined in accordance with method 200.It is noted that method 200, optionally rotates an object so that agroup vector of the object having a maximum component perpendicular tothe z-axis, is parallel to the y-z plane, which generally results in anarea of a projection of the object on the y-z plane being a minimum.

Let P_(yz)(i) be the projection on the y-z plane of an i-th object ofthe plurality of objects and let the projection have an external contourcharacterized by a rising edge and a falling edge. FIG. 7A schematicallyshows an exemplary i-th object 300 of the plurality of objects at anarbitrary location relative to coordinate system 21 and having a y-zprojection 302, characterized by a rising edge 304 and a falling edge306. Rising edge 304 begins at a first point y_(iR) along the y-axis andfalling edge 306 ends at a point y_(iF) (y_(iF)>y_(iR)) along they-axis. Define a lower y-bound (LYB(i) for the i-th object, e.g. object300 in FIG. 7A, which is an average value of the y coordinate of the y-zprojection P_(yz)(i) over all layers along its rising edge 304.Similarly, define an upper y-bound (UYB(i) for the i-th object, which isan average value of the y coordinate of the y-z projection P_(yz)(i)along its falling edge 306.

Optionally, in order to increase similarity between the profilesP_(yz)(i) of all parts, each i-th object is oriented in a “preferredorientation” for which, a “rising edge coordinate difference”,Δ_(iR)=(UYB(i)−y_(iR)) is less than or equal to a “falling edgecoordinate difference” Δ_(iF)=(UYB(i)−y_(iF)). In FIG. 7A object 300 isshown in its preferred orientation.

An example shown in FIG. 7C demonstrates the advantage of increasedsimilarity brought about by “preferred orientation” in reducing theaverage Y span of two parts (i, j).

In accordance with an embodiment of the invention, if a rising orfalling edge of a contour has an undercut, i.e. it is at least doublevalued for at least some y-coordinates, the contour is modified toremove the undercut by replacing at least a portion of the contour witha line parallel to the z-axis. The lower and upper y-bounds for theobject are determined from the rising and falling edges of the modifiedcontour. FIG. 7B schematically shows an i-th object 310 having a y-zprojection P_(yz)(i) 312 characterized by undercuts 314. The figure alsoshows how the projection is modified be replacing portions of itscontour with line segments 316, shown dashed, which are parallel to thez-axis to remove the undercuts, in accordance with an embodiment of theinvention. The reason for removing the undercuts is that when thesupporting structure is added to the objects, the support fills out theundercuts.

To arrange the objects on construction platform 24 in accordance with anembodiment of the invention, the objects are “positioned” on theconstruction platform in descending order of their height or “Y×Z”figure, while maintaining each object in its preferred orientation. Afirst object having a greatest height or “Y×Z” figure is firstpositioned on production platform 24 so that its x-y projection on theplatform, i.e. the x-y plane, remains totally within the platform andthe x and y-coordinates of the centroid of the x-y projection areminima. The object is thus located in the corner, hereinafter the“starting corner”, of the platform 24 having a minimum x value and aminimum y-value.

Thereafter, an i-th object, i>1, is placed on construction platform 24as follows. A projection on the y-z plane, hereinafter referred to as a“y-z union projection, UP_(yz)(i−1)”, which is a union of the y-zprojections of all (i−1) previously positioned objects and a contour, a“y-z union contour”, of the union projection are determined. An x-yunion projection, UP_(xy)(i−1) of the x-y projections on the x-y planeof all the (i−1) previously placed object is also determined. It isnoted that a union projection UP_(yz)(i−1) of (i−1) objects is theshadow that all the (i−1) objects cast on the y-z plane. Objects orparts thereof that are shadowed by other objects do not contribute tothe union projection. Said “y-z union contour” is processed to determinea union lower y-bound, ULYB(i−1), and a union upper y-bound, UUYB(i−1)according to the same method specified before for LYB(i) and UYB(i),with the following exception: The average on minimum (maximum) ofy-value of the rising (falling) portion of the contour is performed onlyup to the full height of the i-th object. A y-span of the unioni−1UY_SPAN is defined as

UY_SPAN(i−1)=UUYB(i−1)−ULYB(i−1).

In addition we specify UUYB′(i) and ULYB′(i) in a similar way of UUYB(i)and ULYB(i), with the exception that the average is done up to theheight of the i-th object (and not i+1), and

UY_SPAN′(i)=UUYB′(i)−ULYB′(i).

Optionally, starting from the starting corner of construction platform24 (for which the x and y-coordinates are minima) the i-th object isthen moved along the x-axis, and as required along the y-axis, todetermine a smallest x-coordinate and smallest y-coordinate for theobject's centroid for which:

-   -   a) its x-y projection P_(xy)(i) does not overlap x-y-union        projection UP_(xy)(i−1), i.e. the intersection        UP_(xy)(i−1)∩P_(xy)(i)=0; and    -   b) the number of printing scans required for the union of the i        parts is not larger than that of the i−1 parts. This may be        achieved (when possible) by checking that the y-z projections,        UP_(yz)(i−1) and P_(yz)(i), substantially include one another up        to the height of i-th part, or at least, [UY_SPAN′(i) modulus        SYS]=[UY_SPAN(i−1) modulus SYS]

In condition b) above, SYS is the shuttle Y-span defined above and issubstantially equal to a widest swath that shuttle 28 can print insingle x-scan pass along the x-axis.

The priority of said various requirements are as follows: First: a) andb). Second: x coordinate of the centroid is minimum. Third y coordinateof the centroid is a minimum

If a location for the i-th object cannot be found that satisfiesconditions a-v, then, if possible, the object is placed on constructionplatform 24 at a smallest x-coordinate and smallest y-coordinate forwhich U_(xy)(i−1)∩P_(xy)(i)=0. Alternatively the object is placed whereUY_SPAN′(i) modulus SYS is a minimum.

If the latter is not possible, the process described above is repeatedin an attempt to place an (i+1)-th object of the plurality of objects onconstruction platform 24 together with the (i−1) objects already inplace. If the process fails again, the process is repeated for the(i+2)-th object and so on until the plurality of objects planned forproduction on the construction platform is exhausted.

The following examples may demonstrate the advantage of said method:

FIG. 8 shows two parts, a part 800 which is composed of a low portion802 and a high portion 804, and a part 806 which has only a highportion. Traditionally the placement method optimized placement onlyaccording to the projection boundary of the objects on X-Y plane (exceptfor determining the order according to height). The only two criteriawere: a. Placing the objects as close to each other as possible; this isin order to minimize scan area, and b. Having the centroid of theprojections at as small Y as possible; this is in order to minimize thenumber of scan passes. The value of Y length of the head (SYS) wasdisregarded. In addition, traditionally the pass locations in Ydirection were fixed. The only variation was that if, at a certainlayer, the objects filled an area that was included only in passes fromi to j, the passes outside the i to j range were not scanned; but thelocation of i, . . . j passes remained the same as in the lowest layer.This traditional placement is shown with part 806 indicated by reference806′.

According to some embodiments of the invention part 806 is placed in they-z “shadow” of part 800, as indicated by reference 806. In addition,according to this invention, the pass location may vary from layer tolayer in response to the object geometry along Y direction, and isoptimized for minimum scan time of the layer, which is substantiallyachieved by minimum number of passes.

The pass location when scanning the high layers is as shown in thefigure (dashed horizontal lines). The Y position of the head whenscanning the high layers is shown at 808. It is clear from the figurethat the number of scan passes when scanning high layers has changedfrom two scans, according to the traditional method, to a single scan.Note that scanning the lower layers has not changed and is provided bythree scan positions indicated as 810.

It is understood that part 806 need not be limited to only a highportion, but that the same principles apply if both parts 800 and 806have high and low parts. In general, the parts are matched to each otherso that a minimum number of (optionally specially positioned) scans areused for the two part ensemble.

FIGS. 9A and 9B show the y-z union contour 902 of i−1 objects and they-z contour 904 of the i-th object.

FIG. 9A demonstrates the traditional placement, in which object i isplaced at minimum Y value. As a consequence a three pass scan isrequired substantially for all layers before part i is almost done.

It should be noted that different scan regimes (positioning of thesuccessive scans) may be used for different layers, and that the numberof scan regimes is not limited to two such regimes as in the Example ofFIG. 8. Thus, depending on the various heights and extents a first scanregime can be used for the lowest layers in this example, a second scanregime can be used for intermediate layers and a third scan regime usedfor the highest layers.

On the contrary, FIG. 9B demonstrates the placement according to anembodiment of the invention, in which object i 904 is placedsubstantially within the shadow of the union of preceding objects 902.It is clear from the figure that adding the ith part does not increasethe number of passes for producing all parts together, and only two scanpasses are required for the majority of layers before part i is done andonly a single scan is required for the highest parts of the scan. Thisis true when we assume that the pass location may vary from layer tolayer as discussed above.

It is noted that for the example of FIGS. 9A and 9B, the levels lowerthan a point indicated by reference Z′ are scanned three times, with thefirst scan originating at Y=0. For the higher parts (up to Z″) thestarting point for the first scan is at Y=Y′. A third scan regime foreven higher portions starts at point=Y″. This third scan regime includesonly a single scan. The positioning of the ith object is not critical,and may very from the illustrated position in FIG. 9B to the right, upto the dashed illustrated position in the figure.

In some embodiments of the invention, an RPA similar to RPA 20 iscontrollable to operate in a number of different production modes thatprovide options to produce a same object at different costs and/orproduction times and/or quality. For each production mode, at least oneparameter that affects production, e.g. a parameter that affectsproduction time, and/or quality and/or cost, is set to satisfy aspecification of the production mode.

Optionally, the RPA is configured to operate in a production modeselected from a fixed number of preset production mode options, forexample a draft, standard and high quality production mode. When a givenproduction mode is selected, the RPA automatically adjusts productionparameters to meet the specifications of the production mode. Forexample, to produce an object in a draft mode, the RPA may configureitself to print relatively thick construction layers, and/or scan alongthe x-axis at a relatively high speed, and/or reduce the amount ofmaterial removed by a planing roller from freshly printed constructionlayers. Thickness of a construction layer is optionally controlled bycontrolling a ratio of a rate of deposition of SM and CM material fromoutlet orifices 132 to a scan velocity of shuttle 28 along the x-axis.Rate deposition is optionally controlled by controlling an orificeinjection voltage, or preferably an orifice injection rate. UV power forcuring deposited SM and BM and flow rate of air for cooling curedmaterial are adjusted to match the deposition rate and scan velocity. CMtemperature also has impact on both injection layer thickness and buildquality. The higher the temperature the larger the drop volume andtherefore the larger the layer thickness, but the lower the buildquality. Therefore CM temperature is also a parameter that is optionallyadjusted according to the selected production mode.

Additionally or alternatively, the RPA is optionally configured toenable a user to adjust production parameters to tailor cost, productiontime and quality to the user's requirements. Optionally the RPA presentsan interface to the user displaying value ranges for productionparameters that he can adjust to meet the requirements. When aparticular value is selected by the user for a given productionparameter, the controller adjusts the ranges of parameters whose valuesremain to be determined, to limit the user's choices for the remainingparameters to values compatible with those already chosen.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments will occurto persons of the art. The scope of the invention is limited only by thefollowing claims.

What is claimed is:
 1. A method of simultaneously producing a pluralityof objects by sequentially printing thin layers of construction materialone on top of the other in a stacking direction Z on a surface of aconstruction platform, the method comprising: determining a buildconfiguration for producing the plurality of objects which defines aposition and orientation of each of the plurality of objects on theplatform; and producing the plurality of objects by printing theconstruction material in a first pass parallel to a scanning direction Xand, as required, in an additional pass parallel to the first passdisplaced therefrom along a displacement direction Y; wherein a Ycoordinate of at least one pass on one layer is different from that ofany of the passes on a lower layer.
 2. The method of claim 1, comprisingupdating the Y coordinate of the at least one pass in the one layerbased on a span in the Y direction of the plurality of objects in theone layer.
 3. The method of claim 1, wherein the Y coordinate of atleast one pass is defined to minimize the number of passes required toprint the construction material for the one layer.
 4. The method ofclaim 1 comprising positioning the plurality of objects on the platformin height order.
 5. The method of claim 4, wherein the plurality ofobjects are positioned in the height order along the X direction.
 6. Themethod of claim 1, comprising: determining projection of each of theplurality of objects on the Y-Z plane; and positioning the plurality ofobjects in descending height order, wherein the height is defined alongthe Z direction.
 7. The method of claim 1, comprising positioning atleast one of the plurality of objects on the platform in an orientationthat has the lowest center of mass.
 8. The method of claim 1, comprisingpositioning at least one of the plurality of objects on the platform inan orientation having a projection in the Y-Z plane spanning thesmallest area.
 9. The method of claim 8, wherein the at least one of theplurality of objects is the object associated with the largest dimensionin the Z direction.
 10. The method of claim 1, comprising: selecting afirst object from the plurality of objects: determining projection ofthe first object in the X-Y plane; determining coordinates of a centroidof the projection in the X-Y plane; positioning the first object on theplatform so that the projection is on the platform and the coordinatesof centroid are minima.
 11. The method of claim 10, wherein the firstobject has the largest height on the platform in the Z direction. 12.The method of claim 11, comprising positioning a second object from theplurality of objects adjacent to the first object, wherein the secondobject has a second largest height on the platform in the Z-direction.13. The method of claim 1 comprising: comparing the projections of theplurality of objects in the Y-Z plane; and positioning each of theplurality of the objects along the Y direction based on the comparing,wherein the positioning is configured to increase overlap of theprojections.
 14. The method of claim 13, wherein the positioning isconfigured to increase overlap along the Z direction.
 15. The method ofclaim 1, comprising aligning a first rising edge of a first of theplurality of objects with a second rising edge of a second of theplurality of objects.
 16. A method of simultaneously producing aplurality of objects by sequentially printing thin layers ofconstruction material one on top of the other in a stacking direction Zon a surface of a construction platform, the method comprising:determining a build configuration for producing each of the pluralityobjects which defines an orientation of each of the objects on theplatform; and positioning a first object with the largest height in theZ direction on the platform; positioning each of the other of theplurality objects on the platform along a scanning direction X in orderof descending height; and producing the plurality of objects by printingthe construction material in a first pass parallel to a scanningdirection X and, as required, in an additional pass parallel to thefirst pass displaced therefrom along a displacement direction Y.
 17. Themethod according to claim 16, wherein each of the other of the pluralityobjects are positioned along the Y direction to maximize overlap of theprojections of each of the plurality of objects in the Y-Z plane. 18.The method of claim 16, wherein the first object is positioned on theplatform in an orientation with the smallest area of projection in theY-Z plane.
 19. The method of claim 16, wherein the first object ispositioned on the platform with an orientation that has the lowestcenter of mass.
 20. The method of claim 16, wherein the plurality ofobjects are positioned on the platform in an orientation associated witha minimum number of passes required to produce the plurality of objects.